PCI: endpoint: Simplify name allocation for EPF device

This commit replaces allocating and freeing the intermediate
'buf'/'func_name' with a combination of 'kstrndup()' and 'len'.

'len' is the required length of 'epf->name'.
'epf->name' should be either the first part of 'name' preceding the '.'
or the complete 'name', if there is no '.' in the name.

Signed-off-by: Rolf Evers-Fischer <rolf.evers.fischer@aptiv.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>